Trends in laser microvia drilling: demand for the technology is healthy, but new systems reveal mostly engineering advances, not breakthroughs.

In December 2000 I noted that there were 15 to 20 companies claiming to provide microvia drilling systems to the PCB and flex markets, but that when the dust settled there would only be a half dozen or so left standing. I further predicted that the U.S. market would be flat at best for the next several years while worldwide growth of laser drilling, especially in Asia, would show a steep growth curve. In June of 2001, I noted "laser microvia drilling is no longer interesting in that it has become a commodity and is not new technology."

Fast forward to the present. Indeed, companies in China and Taiwan primarily are adding drilling capacity at a very high rate, even though it seems there is already too much capacity. According to Sam Sekine, manager of Hitachi's U.S. operations, Hitachi is selling 100 drilling units per month, of which 20 are laser drills. Furthermore, most of these are destined for China and Taiwan with only a few going to Europe and North America. Also look at Mitsubishi, another Japanese company that provides laser drills. It basically has zero presence outside of Asia, yet is a major player in the Asian market.

In this market the name of the game is high volume, therefore speed, speed, speed. Flexibility is not a major factor. Volume manufacturers are looking for ease of use, ease of setup and speed, including handling speed (autoloaders are an important option). Take, for example, Hitachi's current model microvia drills. The LC-1F21E is a one beam, one-panel C[O.sub.2] drill. The company also provides a two-beam, one-panel C[O.sub.2] laser drill, the LC-2F21E, with 40% to 60% higher throughput, and a two-beam, two-panel C[O.sub.2] laser drill, the LC-2F212E, that has 100% higher throughput than the base model. These arc the models that are selling in Asia-absent flexibility for increased throughput. This tradeoff is normal in system engineering and it is rare to find a truly high throughput machine with a lot of flexibility.

There are three primary ways to increase processing speed: By increasing the capability of the laser itself (remember, the laser by itself is only a light bulb and is basically useless unless integrated into a complete workstation), by increasing the usability of the beam through clever optical engineering, and by clever handling of the control hardware and software.

Higher laser power can decrease processing time if the available energy is used wisely. Since only moderate pulse energies are needed to drill small holes in nonreinforced polymers (indeed, high pulse energies can damage materials), higher laser power is best utilized by increasing the laser's pulse repetition frequency (PRF). The total number of pulses required to drill each via can then he delivered more rapidly. Vanadate rather than YAG is advantageous as the laser medium when operating at high PRF, especially since the relatively short upper state lifetime of vanadate means the pulse-to-pulse fluctuations, even at higher PRF, are relatively small. According to Dr. Corey Dunsky of Coherent Lasers, it is also advantageous to be able to vary the energy of each pulse applied to the via. This is possible using Coherent's Aria laser (FIGURE 1). This technique can be used to tune via sidewall angles and profiles. Today's state-of-the-art UV lasers create pulses with energies in the tens of microJoules at PRFs of 100 kHz or higher. As an example, a 50 [micro]m via in standard nonreinforced ABF material requires only about 20-30 microJoules per pulse at the work surface. Conservatively, the laser energy should be twice that to account for optical losses. The Coherent Avia X laser produces about 70 microJoules per pulse at 100 kHz, which is more than enough to split into two beams of equal power, each of which can be directed to a separate drilling head, thereby introducing parallelism (and therefore speed) into the drilling system. Q-Peak Inc. has recently announced a MOPA (master oscillator, power amplifier) laser that delivers 25 W of 355 nm wavelength light in short (5-20 ns) pulses at repetition rates of up to 100 kHz. According to Dr. Peter Moulton, this laser uses a YLF (yttrium, lithium, fluoride) matrix rather than the vanadate matrix, as it is easier to scale to higher powers, and is the highest power output observed to date at repetition rates exceeding 10 kHz.

[FIGURE 1 OMITTED]

Dr. Werner Wiechmann of Lightwave agrees on the power and repetition rate requirements of lasers, but thinks it is just as important that the "second order" considerations are addressed--dynamic laser performance such as the ability to generate short pulse bursts without significant changes in pulse energies compared to the pulse energies under continuous operation. Other requirements include operation in a 24/7 environment without significant power drift, maintenance-free operation over thousands of hours, uniform spot size and shape, and good beam-pointing stability (FIGURE 2) under varying repetition rates and burst mode conditions. Also, minimal unit-to-unit variation is needed in order for system integrators to easily use the lasers in multiple manufacturing tools. Bruce Craig of Spectra Physics believes his company offers the best value for the industry. "We offer a balance of the highest quality microvia capability at the lowest cost while maintaining the throughput necessary for volume production with our Navigator II YHP40 laser." The laser features modularity, ease of integration, superior beam pointing stability and mode quality.

[FIGURE 2 OMITTED]

C[O.sub.2] lasers are still the accepted technology of choice for the majority of today's mainline PCB microvia drilling applications. Though they cannot easily or cleanly penetrate any hut the thinnest copper, they are effective on most common dielectric materials with via diameters between 60 and 200 microns, drilling at rates of several hundred vias per second. Pulse energies from one to tens of milliJoules and peak powers of several kilowatts are typical. New commercial RF-excited Q-switch C[O.sub.2] lasers, such as the Coherent GEM Q-series, produce a sharp spike lasting less than 150 ns, followed by a tail with variable duration of several microseconds. Peak power in the spike can reach tens of kilowatts. Lasers with pulse energies of 0.4 to 3.0 milliJoules have been evaluated for via drilling. According to Dunsky, even in difficult to process FR-4 (because of the in homogeneous nature of the material), the laser's high peak power produces clean ablation of both the glass fibers and the epoxy resin, and low average power minimizes heat affected zones while high PRF increases potential drilling speed.

Beam shaping has become a very enabling technology for microvia drilling. In the case of UV solid-state lasers, image projection of a uniformly laser-illuminated aperture has emerged as the technology of choice. Special optics are used to change a Gaussian-shaped beam to a flat top beam profile at the aperture plane. Projecting an image of the illumined aperture onto the work piece creates a very round spot of uniform intensity. This allows faster processing because percussion drilling can be used rather than the slower trepanning technique.

Faster and more accurate galvanometer beam steering hardware and software are being developed by companies like Scanlab and a few others. In addition, sophisticated control software has been developed by companies like ESI that permit processing on the fly, in which the moving beam never stops during processing and is coordinated between the galvanometers and the motion stages. When drilling vias, sometimes the motion overhead takes more time than the actual laser firing, so this type of process control can significantly speed processing time.

It should be noted that there is an inherent increase in drill speed to be gained just by having the vias become smaller and more densely packed. Smaller vias require less total laser energy for ablation. Also, in general, when vias get smaller, the materials get thinner, thereby also decreasing the total number of laser pulses needed to penetrate a given material. Since move time is a high fraction of the overhead, the fact that vias are closer also gives an inherent increase in drill speed (measured in vias per unit time).

When we consider North America and Europe the situation is somewhat different. There is very little volume work occurring in these theatres. Situations that do not require the manufacture of hundreds or thousands of a part number but instead mandate perhaps hundreds of different part numbers with small runs require more flexibility than speed. Several of the microvia-system manufacturers are promoting their products as more flexible tools capable of being multitasked for a variety of applications such as routing, soldermask repair; bulk dielectric removal and marking.

Stephan Schmidt of LPKF Lasers believes there is a real need in the small- to mid-size board shops for flexibility and the ability to process both copper and dielectrics, perhaps without secondary cleaning operations. UV systems especially are more cost effective if they are able to perform a variety of tasks (FIGURE 3). These laser systems are developed to provide minimal changeover times between very different jobs without compromising quality. LPKF systems are unique in that they use lamp-pumped technology rather than diode-pumped solid-state lasers. While admitting that older lamp-pumped technology suffered from poor beam quality, low pulse stability, frequent maintenance and high downtime among other things, LPKF concludes that recent advances have eliminated all but the lamp replacement issues, with the added advantage that the lamp-pumped lasers generally require a lower initial investment and lower operating costs. Dave Howard of Excellon suggests that its Cobra machine is a differentiator as it is a hybrid UV/C[O.sub.2] system. While Hitachi also provides a hybrid machine, the Excellon tool is sold at a much lower price and is built and maintained in the U.S. Other features include a micromachining capability, allowing the tool to be used for a wide variety of applications, and con current drilling of both lasers simultaneously to improve process time. Also being added is a "multi-pass" feature that allows the user to serially hit the target with both lasers sequentially, as is required for instance for stacked vias. Another interesting example of multitasking is found in a specific (yet to he named) company that is developing a tool to be used both for microvia drilling and also for trimming embedded passives. The thought is that the overhead required to bring two tools in house to do a specific task is too much for small- to medium-sized companies, so to be more cost-effective a multi purpose tool is under development.

[FIGURE 3 OMITTED]

A few parting comments. First, GSI Lumonies was the first company to offer a dual laser system using the C[Osub.2]-TEA laser. This type of C[O.sub.2] laser is usually slower than a Gaussian laser, but has a lilt of virtues including short pulse length. We have found this laser to be very useful for many applications that require gentle processing, but with a C[O.sub.2] laser. One of the drawbacks is that the PRF is measured in hundreds of Hz, not tens or hundreds of kHz, but it is still a very useful tool. It is surprising that GSI does not seem to be very aggressive in marketing this, or any other microvia drill, at present. Second, with respect to Europe, it seems that the larger companies are more reluctant to outsource processing, at least compared with the U.S. Therefore, we see companies like Siemens developing and marketing their own microvia drills, both for in-house use and also to offer as a commercial product. This is somewhat true in Asia also; for instance, it is clear that Hitachi uses its microvia drills internally. According to Sekine, new technology from Hitachi will be appearing soon. I expect it may have something to do with its acquisition last year of NanoVia and the latter's patented beam shaping technology.

In conclusion, laser microvia drilling is certainly a very healthy market at present, Inn in general only engineering advances have been made and marketed the last few years and a real breakthrough technology has not yet been developed and marketed. Perhaps a breakthrough is just around the corner, and it will be interesting to see what a resurgence of the economy brings. PCD&M

RONALD SCHAEFFER is CEO of Photomachining (photomaching.com). He has a Ph.D. in chemistry from Lehigh University and more than 15 years experience with lasers. He may be contacted at [e-mail].